1. Field of the Invention
The invention relates in general to a structure of a stacked barrier layer in multilevel interconnects, and more particularly to a structure of a stacked titanium nitride barrier layer in multilevel interconnects.
2. Description of the Related Art
In the high density integrated circuit process titanium nitride is the most prevailing material for forming a barrier layer. The barrier layer is used for deterring the diffusion between alumnum and silicon to eliminate spiking and electromigration. In multilevel interconnect process, a barrier layer is preferrably composed of titanium nitride and titanium (TiN/Ti). A metal plug is composed of the barrier layer and alloys.
A conventional structure of a plug in multilevel interconnects is illustrated in FIG. 1. First, a semiconductor substrate 100 is provided and a source/drain region 101 is formed in the semiconductor substrate 100. A dielectric layer 102 is formed on the semiconductor substrate 100 to insulate metal layers. Photolithography process is performed to pattern the dielectric layer 102 and a contact opening 104 is formed in the dielectric layer 102 to expose the source/drain region 101 . A titanium layer 106 is formed on the contact opening 104 using physical vapor deposition (PVD), for example, DC magnetron sputtering. The titanium layer 106 is about 200-1500 .ANG. thick. A titanium nitride layer 108 is formed on the titanium layer 106 using physical vapor deposition (PVD). The titanium layer 109 is about 800-1200 .ANG. thick. A plug 110 is formed in the contact opening 104. The method of forming the plug 110 is first depositing a metal layer, for example, alumnum to fill the contact opening 104 and etching back to form the plug 110. Then the wafer is cleaned for subsequent metal interconnects process.
FIG. 2 showing a columnar structure of the titanium nitride layer. There are spacings between the columnar titanium nitride grains. Alumnum metal can diffuse along the spacings to react with silicon. Therefore, the method to deter the diffusion between alumnum and silicon is to elongate or to turn the diffusion path from alumnum to silicon. However, as shown in FIG. 3, the physical vapor deposition (PVD) is no longer a good method to form a titanium nitride layer and can not match the need that the size of devices decrease. It is difficult to fill the Ti/TiN layer 118 in the contact window 104 with high aspect ratio by PVD because of its poor step coverage. Therefore, the leakage current increases.